With a product such as a supercharger, a gas turbine, or an industrial compressor, “an increase in a flow rate” is an important challenge in improving performance. The term “increase in the flow rate (increase in the capacity)” of a centrifugal compressor refers to increasing a discharge flow rate in the compressor of the same shell size. Generally, the outer diameter of an impeller is used as a reference dimension. In other words, the increase in the flow rate refers to increasing the discharge flow rate in the impeller of the same outer diameter.
As a mutually exclusive event for this “increase in the flow rate”, “a decrease in efficiency” poses a problem. A “technology for achieving an increased or large flow rate while suppressing a decrease in efficiency” is very meaningful in the industrial field.
On the other hand, “an increase in pressure ratio” is an important technical requirement. This is because the increased pressure ratio can lead to a high output and a high efficiency with a small reciprocating engine in a supercharger (turbocharger) to which a centrifugal compressor is applied. In a gas turbine as well, the increased pressure ratio enables a high output and a high efficiency to be obtained with a small engine. In a supercharger, in particular, when the required pressure ratio is increased to 4 to 5, there is a simultaneously growing demand for the increased flow rate. With such a centrifugal compressor having a high pressure ratio, a decrease in the efficiency associated with the increase in the flow rate is marked. Thus, the “technology for achieving an increased or large flow rate while suppressing a decrease in efficiency in a centrifugal compressor having a high pressure ratio (4 to 5)” is of industrially significant importance.
Non-Patent Document 1: Transactions of the ASME 126/Vol. 110 JANUARY 1988